Such an embodiment of the stem in two assembled parts, namely the insert and the actuation tube, is intended to allow the mounting of the piston on the stem, for example when said stem is provided with shoulders facing the base particularly for limiting the stroke of the piston or for forming a seat for an elastic member suitable for returning the stem to the far position.
In a known pump of the aforementioned type, the insert is rigidly attached to the actuation tube, by sleeve-fitting by force or other means.
Such a pump poses problems in terms of fabrication because it requires a precise relative positioning of the insert and of the actuation tube and requires the insert and the actuation tube to be manufactured with reduced tolerances.
In addition, the known pump has piston sealing and wear problems. Specifically, such a pump may have an angular offset between the insert and the actuation tube. As a result, the piston cannot exert a distributed pressure on the inside of the body which adversely affects the sealing and increases the risks of premature wear of the piston. And the annular surfaces of the base and of the piston cannot come into contact in a uniform manner. The result of this is a lack of sealing of the discharge valve and a risk of wear of the piston surfaces.